For example, as an ion source used in a mass spectrometer, there has been known one that sample molecules are ionized by impacting electrons. In such an ion source, it is necessary that, by controlling thermo-electrons by causing a potential difference between a thermionic emission filament and a mesh for accelerating the electrons, the thermo-electrons appropriately accelerated are supplied to an ionization chamber.
This is because, for example, in the case of using argon as a sample, since it becomes difficult to analyze the argon sample if divalent ions are generated by strong impingement with thermo-electrons, it is suitable to impinge the thermo-electrons against the argon sample with a relatively small acceleration and, in this way, a suitable condition should be considered depending on a sample.
Further, since it is necessary to extract ions generated by the ion source from the ion source as an ion beam suitable for separation and analysis, it is also necessary to adjust a potential gradient in the ionization chamber to be optimized.
However, in a conventional ion source, as disclosed in Patent Literature 1, since an entire side wall of the ionization chamber is formed of a mesh, it is liable that an external electric field caused by a potential of a thermionic emission filament pass through the mesh and permeate into the ionization chamber, which may influence the electric field in the ionization chamber.
Therefore, in the case where the potential of the thermionic emission filament is changed in order to realize an optimal thermo-electron supply, the external electric field is changed and the changed electric field may possibly permeate into the ionization chamber to thereby also change the electric field in the ionization chamber. If the electric field in the ionization chamber is changed, it is necessary to adjust also a potential of an ion extraction electrode and adjust the potential gradient in the ionization chamber so as to easily extract the ions. If not so, there may arise a problem that the ions cannot be efficiently extracted from the ionization chamber.
Furthermore, in the case where the potential of the ion extraction electrode is changed to obtain an ideal potential gradient in the ionization chamber, unless the potential of the thermionic emission filament is adjusted to adjust also the permeation of the external electric field into the ionization chamber, it is liable that the ions cannot be efficiently extracted. In addition, by changing the potential of the thermionic emission filament, it is also liable that suitable thermo-electrons cannot be supplied to the ionization chamber.
That is, since the potential of the thermionic emission filament and the potential of the ion extraction electrode are in a subordinate relationship to each other, there has been a problem that it is difficult to simultaneously make both an ideal thermo-electron supply and a potential gradient in the ionization chamber compatible.